1. Field of the Invention
The present invention relates to, for example, a NAND flash memory in which a plurality of bits are stored in a memory cell, and more particularly, to a nonvolatile semiconductor storage device for converting write data.
2. Description of the Related Art
In recent years, a multi level memory in which information of a plurality of bits can be stored in a memory cell by setting one of a plurality of threshold voltages corresponding to a plurality of write data in the memory cell has been developed. Further, a write operation in a flash memory is roughly constituted of a program operation for applying a write voltage to a memory cell, and a verify operation for verifying write completion. In the case of a multi level memory, it is necessary to perform a plurality of program operations and verify operations in accordance with write data.
For example, a memory cell which can store information of three bits holds one of eight states. Thus, a verify operation is needed for each of the eight states. Accordingly, in the overall write operation, the number of times of the verify operations becomes larger than the number of times of the program operations for applying the write voltage, thereby requiring a long time for writing data (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-192789). Therefore, a nonvolatile semiconductor storage device in which a writing speed can be made high when a plurality of bits are written to a memory cell is desired.